This invention relates to a ductile copper-niobium alloy, to a method for preparing the alloy, and to a method for the commercial production of composite superconducting wire.
One present method used commercially for preparing multifilament composite superconducting wire such as Nb.sub.3 Sn in a predominately copper matrix consists in mechanically drilling a large number of holes in a copper or bronze billet, inserting a rod of niobium into each hole and extruding, swaging and drawing the billet in several steps until the niobium rods are reduced to the desired filament size. The wire must then be reacted in order to form Nb.sub.3 Sn. This process is described in detail in an article by Eric Gregory in Manufacture of Superconducting Materials, R. W. Meyerhoff, Editor, American Society for Metals, 1977. The process is expensive and exacting and the size generally limited to filaments larger than 2 .mu.m in diameter.
In another method, cylindrical rods of niobium are inserted into tubes of normal metal, such as copper, to form a composite rod. A large number of the composite rods, which are hexagonal in cross-section to improve packing density, are then tightly packed into an extrusion can of normal metal, sealed and reduced in cross section by various methods of hot and cold working to produce a multifilament composite wire containing elements of superconducting material in a matrix of normal material. Preparation of the composite rods is difficult and time consuming because of close tolerances necessary to ensure a good bonding between the metals and to prevent contamination. The hexagonal outer surface of the tubes must be dimensionally accurate so that large numbers of the rods can be tightly packed into the extrusion can to prevent trapping of gas or other contaminants between the rods which would affect the superconductor. Thus preparation of composite superconducting wire by this method is time-consuming, expensive and necessitates the acquisition of components which are expensive to meet the tolerance requirements.
To overcome these difficulties, attempts have been made to grow filaments directly from a melt in the form of long fibers or dendrites. One such alloy, described in U.S. Pat. No. 3,817,746, consists of randomly distributed superconducting particles or short filaments embedded or disposed in a ductile non-superconducting matrix which can then be worked to form composite superconducting wire. However, the resulting wire is not completely satisfactory for the preparation of superconducting components such as magnets or the like.